Color reaction between electron donating leuco dyes and electron accepting compounds is well known and has been embodied into two-component-system recording materials, such as pressure-sensitive paper, heat-sensitive paper, photo- and pressure-sensitive paper, electrothermal recording paper, and the like. References can be made to it, e.g., in British Pat. No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, Japanese Patent Publication No. 23922/85 and Japanese Patent Application (OPI) Nos. 179836/82, 123556/85 and 123557/85 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Performance properties that should be fulfilled by these recording materials include (1) sufficient color density to be developed and sufficient color development sensitivity, (2) freedom from fog, (3) sufficient fastness of a developed color image, (4) appropriate hues form when developed and suitability for use on copying machines, (5) high S/N radios, (6) sufficient chemical resistance of a developed color image, and the like. However, none of the conventional recording materials satisfies all of these requirements.
In particular, heat-sensitive recording materials which have undergone a marked development in recent years have disadvantages of fog due to solvents, etc., and discoloration or decoloration of a developed color image due to fats and oils, chemicals, etc. That is, upon contact with stationeries or office supplies, e.g., aqueous ink pens, oily ink pens, fluorescent pens, inkpads, adhesives, pastes, diazo developers, etc., cosmetics, e.g., hand creams, milky lotions, etc., the white background develops a color or a color developed area undergoes discoloration to thereby seriously impair commercial value.
In order to solve these problems, efforts have been made by providing a chemical resistant protective layer, etc., as disclosed in Japanese Patent Publication No. 27880/69, Japanese Patent Application (OPI) Nos. 30437/73 and 31958/73, etc. However, provision of a protective layer not only causes reduction in color development sensitivity, sticking or noise on recording due to insufficient matching with a thermal head of a heat-sensitive recording device, blotting with an ink due to poor writing properties and the like, but also makes the production process complicated.
Further, various attempts have been made to improve stability of a developed color image as disclosed in Japanese Patent Publication No. 43386/76, Japanese Patent Application (OPI) Nos. 17347/78, 72996/81 and 194891/84, British Patent Publication No. 2,074,335A, etc However, the stabilizing effect attained is still unsatisfactory, or if any effect being obtained, the white background undergoes color development (i.e., fog).